1. Field of the Invention
The present invention relates to an apparatus and a method for simulating optical amplifying characteristics of an optical amplifier applied to an optical transmission system, and in particular, to an optical amplification characteristics simulation apparatus and an optical amplification characteristics simulation method considering the gain fluctuation due to a gain spectral hole burning phenomenon.
2. Description of the Related Art
As shown in a basic configuration diagram of FIG. 27 for example, an optical amplifier applied to a wavelength division multiplexing (WDM) optical transmission system comprises: an optical fiber (EDF) 102 doped with erbium ions (Er3+) serving as an amplification medium; pumping light sources 102 for pumping the EDF 101; optical couplers 103 for multiplexing a signal light and a pumping light; a gain equalizer 104 for flattening a gain or an output power relative to a wavelength; and optical isolators 105 arranged on the former stage of the input side optical coupler 103 and on the latter stage of the output side optical coupler 103.
Heretofore, as shown in FIG. 28 for example, amplification characteristics of the optical amplifier as described above have been calculated by a simulation apparatus using a model in which an amplification band is regarded as one band, and wavelength characteristics are unambiguously determined according to a value of a population inversion rate which is defined by the number of ions contributing to the amplification among the total ion numbers. According to such conventional simulation, the population inversion rate is replaced with gain wavelength characteristics. Namely, there has been used the approximation in which the wavelength characteristics of the gain or the output power of the amplification medium of the optical amplifier which is automatically gain controlled is not changed even if wavelength numbers (the signal channel numbers), the wavelength allocation (the signal channel allocation) or the input power level is changed.
In recent years, there has been promoted the introduction of a WDM optical transmission system provided with nodes as shown in FIG. 29 having wavelength routing functions such as optical add/drop multiplexing (OADM), optical cross-connecting (OXC), optical hub (HUB) and the like, and accordingly, there is caused a possibility that the wavelength numbers and the wavelength allocation are significantly changed due to the wavelength routing. FIG. 30 shows a configuration example of a conventional OADM node.
In such a WDM optical transmission system, when a failure, such as the transmission path breakage or the like, occurs, measures are taken in which the line switching is made for suppressing the degradation of the service quality so that the interruption of signal transmission is suppressed to a minimum. To be specific, in the above system shown in FIG. 29, considering the case where a failure occurs in a transmission path between an OADM node #1 to which a signal light of M waves is added and an OADM node #2 to which a signal light of N waves is added, a line which transmits the signal light of M waves added at the OADM node #1 to an OADM node #n is switched from a clockwise direction to a counterclockwise direction due to the failure. In this case, if the setting of the wavelength numbers is, for example, M=39 and N=1, the wavelength numbers of the signal light sent from the OADM node #2 to an OADM node #3 are changed from 40 waves to one wave around the failure. The optical amplifier applied to each node of such a WDM optical transmission system is required to have the properties in which the wavelength characteristics of the gain or the output power are flat, even if the wavelength numbers and the wavelength allocation are significantly changed.
However, it is understood that it is hard to maintain the flatness of the gain wavelength characteristics or the output power wavelength characteristics by the conventional optical amplifier applied with the automatic gain control, and as one of the reasons thereof, it is considered a gain spectral hole burning (GSHB) phenomenon of rare-earth ions. The gain spectral hole burning phenomenon means the spectral hole burning caused by an amplification phenomenon due to the stimulated emission. For example, for the gain spectral hole burning phenomenon of an erbium-doped fiber amplifier (EDFA), it has been reported that, by controlling a pumping light for holding a gain of a signal light of 1560 nm constant, the gain spectral hole burning phenomenon can be markedly observed (refer to literature 1: M. Nishihara, Y Sugaya and E. Ishikawa, in Proceedings of Optical Amplifiers and Their Applications, OAA2003, Tud.3 (2003)).
Spectrums in the gain spectral hole burning phenomenon can be obtained by getting a difference between gain spectral which are measured by the wavelength sweep of a probe light of lower power after entering a saturating signal light (a signal light by which the gain is saturated) and the probe light into the optical amplifier and gain spectrums which are measured by the wavelength sweep of the probe light of lower power after entering only the probe light. In the difference spectrums, as shown in FIG. 31 for example, a gain difference appearing in a shape approximating a Gaussian function can be observed in the vicinity of wavelengths λ1 to λ6 of the saturating signal light and in the vicinity of 1530 nm thereof, and this difference spectrums are defined as the gain spectral hole burning. The gain difference appearing in Gaussian-like is called a hole, and the hole in the vicinity of the wavelengths of the saturating signal light is called a main hole and the hole in the vicinity of 1530 nm is called a second hole.
For example, in the case of a C-band (conventional band) of the EDFA, the second hole in the vicinity of 1530 nm does not depend on the wavelengths of the saturating signal light to appear at 1530 nm, and the hole-width and hole-depth thereof does not depend on the wavelengths of the saturating signal light. On the other hand, the main hole has characteristics in which the hole-width thereof is approximately constant without depending on the wavelengths of the saturating signal light whereas the hole-depth thereof becomes deeper as the wavelength of the saturating signal light becomes shorter (refer to FIG. 31). Further, in the above optical amplifier, in the case where the saturating signal light is wavelength division multiplexed, it is known that the depth of the second hole is decreased as the total input saturating signal light power is increased whereas the depth of the main hole depends on a rate between the total input saturating signal light power and the input power of each wavelength, and the saturating signal light power.
Thus, as a result that the observation of the gain spectral hole burning phenomenon has been promoted, it becomes possible to perform the simulation of the optical amplification characteristics using the calculation model thereof, and accordingly, a proposal has been made on a simulation apparatus for appropriately restricting an amplified wavelength range, an input/output power range and a gain setting range and for setting a calculating parameter, so that not only experimental results of one wave amplification characteristics but also experimental results of wavelength division multiplexing amplification characteristics can be traced (refer to literature 2: International Patent Publication 2005/002009 pamphlet).
Usually, in the optical amplifier, the length of the amplification medium and an optical circuit configuration are determined in accordance with the required specification, such as the amplification band, the input/output power range, noise characteristics and the like. Therefore, for the calculation model used in the above conventional simulation apparatus, it is also necessary to prepare the calculating parameter for the gain spectral hole burning phenomenon for each required specification.
However, in the conventional simulation apparatus, in the case where the calculating parameter setting does not meet conditions which has been set when the calculation model for the gain spectral hole burning phenomenon is constructed, a calculation result thereof does not always trace actual characteristics. In particular, in the case where the wavelength band, the temperature and the input power range are changed, it is difficult to apply the existing calculation model. Namely, the conventional simulation apparatus has disadvantages in that the calculation model without the versatility is used.
In order to enhance the versatility of the calculation model, to be specific, in order to enable the optical amplification characteristics simulation considering the gain fluctuation due to the gain spectral hole burning phenomenon even in arbitrary amplified wavelength ranges, the arbitrary wavelength numbers and wavelength allocation, arbitrary input/output power ranges, arbitrary gain ranges and arbitrary temperature ranges, it is necessary to construct a new calculation model based on a physical phenomenon. Ideally, it is desired to realize a calculation model in which the gain fluctuation due to the gain spectral hole burning phenomenon can be simulated based on a calculating parameter on the basis of the property and structure of the amplification medium.
Further, in the WDM optical transmission system as shown in FIG. 29, the number of nodes having the wavelength routing functions tends to increase, and as a result, the number of optical amplifiers used in the system are increased. In each optical amplifier, in the case where the wavelength numbers of the signal light and the wavelength allocation thereof are significantly changed as described in the above (for example, from 40 waves to one wave, and the like), as shown in the time-dependence of the power change of the residual signal light in FIG. 32, immediately after the changes of the wavelength numbers and the wavelength allocation, there occurs a phenomenon called an optical surge in which an output power level is instantaneously changed to be increased. The optical surge occurring when the wavelength numbers are changed can be suppressed by taking measures, such as the application of a high-speed automatic gain control (high-speed AGC) system. After the control for the optical surge suppression is stabled, there occurs the output power deviation in which the output power level per one wave is increased or decreased relative to a state before the wavelength numbers are changed. The above gain reduction due to the gain spectral hole burning phenomenon is one factor of the output power deviation after the optical surge suppression. To such output power deviation, a required output power level is maintained by performing an automatic level control utilizing a variable optical attenuator incorporated in the wavelength routing device.
However, generally, it takes several milliseconds or several ten milliseconds until the variable optical attenuator to be utilized for the automatic level control is operated. The above described output power fluctuation which occurs until the automatic level control is effectively performed is accumulated in each optical amplifier disposed on the system, and an accumulative amount thereof is increased as the disposed number of optical amplifiers is increased. Therefore, it is anticipated that the transmission quality is degraded due to the optical output power fluctuation or the optical signal-to-noise (SN) ratio fluctuation.
Thus, in order to ensure the reliability of the optical transmission system, it is necessary to consider the optical output power fluctuation and the optical SN ratio fluctuation. However, there is a problem in that the analysis or the cause clarification has not been sufficiently performed on a phenomenon called the above output power deviation and the accumulation thereof. Namely, heretofore, only the gain spectral hole burning phenomenon of the optical amplifier has been noticed as the cause of the output power deviation, and accordingly, for the simulation thereof, the above described problem is not still solved. In particular, in the conventional simulation apparatus, the detailed design information, such as the optical circuit configuration of the optical amplifier, the pumping control method and the like, is required. Optical parts configuring the optical amplifier are often provided from external parts manufacturer. In such a case, it is difficult for a system designer to obtain the detailed design information of the optical amplifier, and consequently, it becomes hard to make a design considering the optical output power fluctuation and the optical SN ratio fluctuation on the total optical transmission system.